The present general inventive concept relates to apparatuses for measuring performance of a coherent optical receiver and, more particularly, to an apparatus for measuring performance of a coherent optical receiver which measures a common mode rejection ratio.
With the continuous increase in communication capacity, a direct detection method has been conventionally used in an optical communication system of 100 Gb/s or more to detect an optical signal. However, a coherent detection method is increasingly used in recent years. The coherent detection method may improve spectral efficiency to make ultrahigh-speed transmission available and compensate transmission quality degradation through digital signal processing in an electrical region of a receiving terminal.
An optical receiver is used in a coherent detection method for detecting optical signals. Bandwidth, common mode rejection ratio, and channel skew, which are performance index of a coherent optical receiver, may have a great effect on performance of the coherent detection method. Accordingly, there is a need for a method for measuring performance index of the coherent optical receiver.
A typical coherent optical receiver includes a 90-degree optical hybrid, a balanced optical receiver, and a transimpedance amplifier. Conventionally, bandwidth and common mode rejection ratio of a balanced optical receiver, not a coherent optical receiver, have been measured. While it is relatively easy to measure bandwidth and common mode rejection ratio of a balanced optical receiver itself, it is not easy to measure bandwidth and common mode rejection ratio of a coherent optical receiver to which a 90-degree optical hybrid is connected.
As a method for directly measuring a common mode rejection ratio of a coherent optical receiver, there is a method using a common optical modulator and measuring a differential mode component with a single port input, which is opposite to the case of a balanced optical receiver, and a common mode component with a dual port input through a phase modulator at one side path. However, this method is limited in measuring accurate performance of a coherent optical receiver because a maximum value at each frequency is assumed as a common mode component to be bused in a common mode rejection ratio through repeated tests with phase modulation.